Construction machines are frequently used by its nature in a place whose road surface is in a bad condition as compared with general vehicles. Some types of construction machines employ four-wheel drive or six-wheel drive. However, on a soft ground such as a mine or a construction site, since positional differences between the wheels lead to different friction coefficients between the wheels and the road surface, even the above types of construction machines suffer from the slip of part of the driving wheels, so that a driving torque cannot be transmitted to the other driving wheels. In such a case, most of engine output is used to drive the slipping driving wheel or wheels, so that a sufficient amount of the driving force cannot be transmitted to the road surface, thereby reducing acceleration.
When the driving torque to each wheel is excessively large for the friction force between the road surface and the wheel, the side force of the wheel is reduced by an amount corresponding to the excess of the driving force upon the occurrence of slip. In view of the above, it is required to control the driving torque to each wheel in accordance with a road surface condition so as to change the amount of the driving force transmitted to the road surface from the wheel to be appropriate to the road surface.
As devices for controlling the driving force to the wheels of the above types of construction machines, there have been known a traction control (hereinafter referred to as TCS) device capable of adjusting a brake torque to each wheel (see, for instance, Patent Literature 1) and a differential-lock control device capable of locking a differential in a differential mechanism between right and left driving wheels or between front and rear wheels (see, for instance, Patent Literature 2).
Patent Literature 1 teaches an articulated construction machine including separate front and rear vehicle body frames, in which calculations are made for each wheel to obtain a velocity component in a steady state and a velocity component resulting from a change in a temporal articulated state, the former component being obtained by adding the orbital speed of the vehicle calculated from an articulate angle to the average speed of each vehicle (turning outer wheel) or by subtracting the orbital speed from the average speed (turning inner wheel), the velocity component being calculated from a change amount of the articulate angle. A target speed appropriate to the position of each wheel is calculated by adding the velocity component in the steady state to the velocity component resulting from the change in the temporary articulated state. When a difference between the target speed and the actual speed of the wheel exceeds a predetermined value, brake is applied to the wheel.
Patent Literature 2 teaches a construction machine including an inter-axle differential as a differential device capable of distributing engine output to the front and rear wheels, in which a sign of slip of the front wheels are detected based on the rotation speed of a transmission output shaft, the rotation speed of a front output shaft of the inter-axle differential, and the rotation speed of a rear output shaft of the inter-axle differential. When the sign is detected, a differential-lock amount of the inter-axle differential is controlled.